PEAR Is short for ‘PHP Extension and Application Repository’
PEAR is having it’s primary goal to become a repository for PHP extensions and library code. The most important goal of the PEAR project is to try to define standard coding packages that can be used be developers directoly and it is re-usable coding.
Each package is a seperate project and is having a diffreent developer team.
Packages are in the gzipped tar files with a description file inside, and installed on your server using the PEAR installer.
You can install any pear package as
———–
pear install pear-package-name
———–
You can check all the PEAR packages that are available with
————
pear list-all
————
You can check installed PEAR packages on your server as
————
pear list
————

The PHP Extension and Application Repository, or PEAR, is a framework and distribution system for PHP code components. PEAR project was founded in 1999 by Stig S. Bakken to promote reuse of code that performs common functions.

It is like apt-get for adding packages to your operating system, or CPAN for adding new functionality to Perl libraries.
There is a good introduction on how to install at:
http://pear.php.net/manual/en/installation.php